The present invention relates to methods and means by which titanium alloys can be prepared. More specifically, it relates to a method of preparing rapidly solidified titanium alloys through a melt extraction technique.
Processing titanium metal in the liquid phase is difficult because of the extraordinary degree to which titanium will dissolve almost any other material. It is very difficult to find a container for pure titanium or titanium having relatively low percentage of the order of 15 to 20% of other ingredients. Except for processes such as arc melting of titanium metal in a chilled crucible, until the present time, it has not been possible to contain liquid titanium within a heated solid container without attack and dissolution of at least a part of the substance of the container taking place. One way in which pure titanium or titanium alloy having very high concentration of titanium (i.e. less than 20% of other ingredients) has been processed from the liquid phase is through melt extraction. The melt extraction process results in formation of ribbon or thread of rapidly solidified alloys of titanium. Rapidly solidified titanium alloys have properties which are distinct from the same composition prepared in bulk melt form by more conventional processing.
One mode of melt extraction involves preparing a crucible to contain the titanium and heating only the center portion of the titanium contained within a crucible to the molten state. The edge of a rotating wheel is then brought into contact with the molten titanium and a filament is formed as a small quantity of titanium freezes onto and temporarily adheres to the edge of the spinning wheel.
The titanium in the crucible is molten only at the center and thus the container for the molten titanium is additional titanium which remains solid.
It is known that the rotating disks which are used for rapid solidification of pure titanium and of certain of its alloys by the melt extraction process described above perform poorly in this process. For certain alloys of titanium, particularly those with higher concentrations of titanium, and which contain significant amounts of molybdenum, columbium and tantalum, it has not been feasible to prepare large quantities of rapidly solidified material because the process has been catastyrophically interrupted by a failure of the rotating disk to continue to function. During the rapid solidification processing runs, the filament of such titanium alloys suddenly fails to separate spinning disk. Following such initial irreversible adherence, a thick layer of the titanium alloy rapidly builds up on the disk. The material on the disk edge continues to accrete and digs deeper and deeper into the melt in the crucible. This effectively interrupts the rapid solidification process inasmuch as the filament does not separate from the disk. It has been observed for some such alloys that only small quantities of filament are prepared before such adherence of the alloy to the disc effectively interrupts the process.
For some of the alloys of titanium and particularly those having higher content of other alloying elements the process is not interrupted so rapidly and larger quantities of filament can be made on a repeated basis. The problem which has remained, however, is to find some scheme by which melt extraction of the titanium alloys containing significant amounts of at least one metal selected from the group consisting of molybdenum, columbium and tantalum can be accomplished on a continuous basis. To date, this has not been feasible.
Numerous attempts have been made to establish and to improve on the melt extraction process. Early efforts were made by Robert Maringer, Carroll Mobley and co-workers at the Battell-Columbus Laboratories. The essence of the technique which was developed by these individual was to contact a pool of open metal with a rotating disk. The disk was intended to drag off a thin layer of metal from the pool so that the removed metal freezes to the disk but also detaches from the disk and both phenomena occur almost simultaneously. The edge of the disk which contacted the molten pool was typically tapered with an angle of about 60.degree. to about 120.degree. so that a relatively thin edge of the spinning wheel came into contact with the molten metal. These prior art efforts involved forming a continuous wire-type product through use of a smooth edge disk and also involved forming a discontinuous wire or staple-type of product by use of a disk having notches spaced around the rim which contacts the molten metal.
Two principal types of melt extraction have been described by Maringer and his co-workers. The first is a crucible melt extraction in which a molten pool of metal is contained within a crucible and the extraction disk is located above the pool and rotates in a vertical plane. See FIG. 1 attached. A brushing contact of the edge of the extraction disk with the surface of the pool leads to the disk dragging off the thin layer of metal from the pool as explained above. A second type of melt extraction also described by Maringer is the pendant drop melt extraction. According to this process, a stick of metal to be converted to rapidly solidified fine wire or filament is oriented vertically and the bottom end of the stick is melted by directing an electron beam against the end or by induction heating or similar technique. In this pendant drop melt process, the wheel is located below the stick and it rotates in a vertical plane as in the crucible melt extraction process. See FIG. 2 attached for an illustration of apparatus as used in this pendant drop process.
Some aspects of the melt extraction process are described by Maringer et al in several patents and publications. In U.S. Pat. No. 3,838,185, entitled FORMATION OF FILAMENTS DIRECTLY FROM MOLTEN MATERIAL, Maringer et al describe the crucible melt extraction process. In this patent, there is no discussion of this specific disk material or of desired properties of this material. However, there is a caution and the discussion of disk particulars that column 7, lines 8-21 and on lines 39-63, emphasizing the need to prevent the edge of the disk from getting so hot that the melt extraction process ceases. Thermal conductivitity and the shape of the disk edge are described. The disk materials are disclosed and the examples are copper and aluminum.
In a related patent of Steward et al, U.S. Pat. No. 3,812,901, entitled METHOD OF PRODUCING CONTINUOUS FILAMENTS FROM A ROTATING HEAT-EXTRACTING MEMBER, the crucible melt extraction process is described. Also described is a method of generating a tension on the wire product after solidification. In the examples, a brass disk is disclosed as having been used to melt extract aluminum, a nickel disk to extract HADFIELD steel, a copper disk to extract cast iron, and an aluminum disk to extract low carbon steel. Disk material is not described as of critical significance.
In U.S. Pat. No. 3,861,450, entitled AN IMPROVED METHOD OF FORMATION OF FILAMENT DIRECTLY FROM MOLTEN MATERIAL, Mobley et al. described using a protective atmosphere around a crucible melt extraction disk to prevent sticking. A problem of metal sticking to the wheel occurred and is reported in the patent but the problem is not associated with the composition of the wheel material. The disks used in their examples are water cooled copper and water cooled composite disks of aluminum and copper.
In U.S. Pat. No. 3,871,439, entitled METHOD OF MAKING FILAMENT OF SMALL CROSS-SECTION, Maringer et al. described a method of crucible melt extraction in which the extraction disk is a cylinder with the helical thread on its periphery. The disk materials disclosed are aluminum and copper.
In U.S. Pat. No. 3,896,203, entitled CENTRIFUGAL METHOD OF FORMING FILAMENTS FROM AN UNCONFINED SOURCE OF MOLTEN MATERIAL, Maringer et al. describe the pendant drop melt extraction process. The patentees state that the disk material "need not be of any special material" but advise that it have a high heat capacity or a high thermal conductivity or alternatively that it be internally cooled. See column 6, lines 2-12. The patentees further state that the invention works with heat extraction disks of copper, aluminum, nickel, molybdenum, and iron. See column 6, lines 12-14. The examples described copper disks used to melt extract tin, aluminum oxide, 304 stainless steel, iron alloy N-155, titanium, and columbium. A cooled rolled steel disk was used to melt extract chromium and 304 stainless steel. A molybdenum disk was used to extract bulk columbium.
In U.S. Pat. No. 3,904,344, entitled APPARATUS FOR FORMATION OF DISCONTINUOUS FILAMENTS DIRECTLY FROM MOLTEN MATERIAL, Maringer et al. describe crucible melt extraction in which the disk which is employed has notches formed in its melt touching surface. The result is the production of discontinuous product by the spin casting of the material. The inventors state that the disk material choice and design are not critical. See column 7, lines 11-18. In the four examples presented, the use of a copper disk is disclosed.
In U.S. Pat. No. 4,154,284, entitled METHOD FOR PRODUCING FLAKE and in U.S. Pat. No. 4,242,069, entitled APPARATUS FOR PRODUCING FLAKE, Maringer describes a refinement in disk design in order to permit a discontinuous product to be made. Notching of the disk for use either in crucible melt extraction or in pendant drop melt extraction is disclosed. In the examples, a brass disk is employed for extracting 304 stainless steel, zinc and titanium alloy Ti-6Al-4V. A table identifies copper and A-6 steel as the material of other disks.
The crucible melt extraction and the pendant drop melt extraction processes are described in an article entitled "Casting of Metallic Filament and Fiber", appearing in the Journal of Vacuum Science Technology, vol. 11, No. 6 November/December 1974, pages 1071-1076. In this report, the authors state that "disk materials of a wide variety have been used, including aluminum, copper, various steels, brass, nickel, and molybdenum." They also state that "Fibers have been cast successfully with all disk materials tried." The same general observations on the wide range of suitable wheel materials for use in the crucible melt extraction and pendant drop melt extraction processes was also affirmed in an article entitled "The Melt Extraction of Metallic Filaments and Staple Fiber" by R. E. Maringer et al. in the American Institute of Chemical Engineers Supposium Series, Vol. 74, No. 180 (1978) pages 16-19.
The pendent drop melt extraction process has been used to make quantities of a ribbon of the titanium alloy Ti-6Al-4V and this is reported in an article entitled "Preparation and Properties of Compacts of Melt Extracted Staple Fibers of Ti-6Al-4V Alloy". This article is authored by R. E. Maringer et al. and appears in the American Institute of Chemical Engineers Symposium Series, Vol. 74, No. 180 (1978) pages 111-116. The amounts of the alloy processed through the melt extraction process was sufficient for consolidation and for mechanical testing. The wheel materials employed were copper and brass.
In the literature and prior art references described and reported above, there is no report of the problems which we have encountered in the melt extraction of titanium alloys having a high percentage content of titanium in addition to at least one metal selected from the group consisting of molybdenum, columbium and tantalum.
Further, there is no report in any of his prior art literature of any solution to the problem of the melt extraction of titanium alloys containing rare earths and containing significant amounts of molybdenum, columbium and tantalum, and having a high concentration of titanium.